1. Field of Invention
The present invention relates to a light exposure device and its operation. More particularly, the present invention relates to a light exposure control device suitable for scanners that provides independent excitation signals for triggering each photodetector such as, for example, a charge coupled device (CCD).
2. Description of Related Art
A charge coupled device (CCD) is a kind of semiconductor device. When a CCD is illuminated by a light source, the intensity of the photons is transformed into a quantity of accumulated electric charges. In general, the stronger the intensity of light beam shining on a CCD, the greater the amount of electric charges generated by the CCD will be. Therefore, the amount of electric charges stored inside a CCD will vary according to the intensity of external light. Utilizing the light intensity/electric charge relationship, a vast number of these CCDs can be arranged systematically into an array forming a CCD module. When photons coming from a light source strike a picture, a corresponding light image is formed. If a photosensitivity CCD module is positioned to receive light signals from the image, image data can be captured. After some transformations of the image data, the transformed image data can be used in a number of applications. For example, a scanner can rely on photosensitive CCD modules to extract a color image from a picture. The method of image extraction includes utilizing three CCD photodetector strips, which are sensitive to red, green and blue light respectively, in the CCD module. During light scanning operation, the red, green and blue lights coming from the image are captured by the corresponding photodetectors in the CCD module. The three primary colors of an image captured by the CCD module are then output to a converter for transforming the image into digital data. Next, the digitized data of the three primary colors are re-grouped, re-creating the original image received from the CCD module. The output of image signals from the CCD module is controlled by an excitation signal. Whenever the CCD module receives an excitation signal, one of the three primary colors from the image is captured.
FIG. 1A illustrates the method of excitation for a conventional CCD module. As shown in FIG. 1A, the CCD module 10 has three CCD photodetector strips. In the fabrication of CCD photodetectors, three different photodetectors each having sensitivity for a particular part of the color spectrum can be separately manufactured so that the three primary colors can be registered. Subsequently, when the three primary colors of the image captured by the photodetectors are recombined, the originally exposed color can be reproduced. The extraction of three primary colors red, green and blue are carried out by a red photodetector 100, a green photodetector 110 and a blue photodetector 120 respectively. In operation, when the picture 4 is illuminated by light source 2, exposure signals 6 will be generated and in turn are sent to the CCD module. Since the exposure signals 6 already contain a mixture of the three primary colors, an excitation signal 15 can be simultaneously generated and sent to the red 100, green 110 and blue 120 photodetectors for capturing the corresponding red, green and blue parts of the exposure signals 6. Hence, in a conventional design, signaling lines 105, 115 and 125 can be connected together so that excitation signal 15 can be simultaneously applied to the respective red 100, green 110 and blue 120 photodetectors.
FIG. 1B is a timing diagram showing the relationship between the excitation signal 6 and the CCD photodetector 15 of FIG. 1A. Since the red 100, green 110 and blue 120 photodetectors are connected together, they will be simultaneously triggered by the same excitation signal 15. For example, as shown in FIG. 1B, when pulses 132, 134, 136, 138, 140, 142 and 144 are produced, three photodetectors including red, green and blue will all be triggered. Because each excitation signal has a cycle time T1, the extraction of a particular color from the image cannot be conducted at a time interval greater than T1. In other words, the light exposure, trigger and data extraction cycle for each CCD photodetector must be finished before the end of a cycle T1.
As an illustration, assume that before the pulse 132 is generated, all three photodetectors are in the light-gathering state. Furthermore, assume that the time from the generation of pulse 132 to the beginning of the next pulse 134 represents a full cycle T1. Within the cycle time T1, all three photodetectors have already completed their respective light gathering operations. Therefore, when pulse 132 arrives, all three CCD photodetectors 100, 110 and 120 are simultaneously triggered. Thereafter, if red light needs to be extracted from the image, the red light from the image can be captured by the red photodetector 100. Moreover, the next round of light exposure is carried out within the cycle time T1 marked by pulse 132 and pulse 134. Within the cycle time T1 between pulse 132 and the next pulse 134, again all three photodetectors have completed a photodetection operation. Hence, when the next pulse 134 arrives, all three CCD photodetectors 100, 110 and 120 are simultaneously triggered. Thereafter, if green light needs to be extracted from the image, the green light from the image can be captured by the green photodetector 110. Moreover, the next round of light exposure is carried out within the cycle time T1 marked by pulse 134 and pulse 136. Using similar operational steps, when the next pulse 136 arrives, the blue light from the image can be captured by the blue photodetector 110. By repeating the above steps, three primary colors of an image can be continuously captured and converted to primary color data. Subsequently, when the stream of three primary color data are properly recombined back together, the original color picture is reproduced.
In the conventional method, the same excitation signal is used to trigger red, green and blue CCD photodetectors. Therefore, the period from light exposure to data extraction for each photodetector must not exceed one excitation cycle. From an alternate viewpoint, since all three CCD photodetectors are triggered by the same excitation signal, each one of the photodetectors is constrained to work together with the other photodetectors. In other words, light exposure is limited to a period within one excitation signal cycle. To achieve the image capture within constraints, exposure time of the CCD photodetectors must match the excitation signal. In a world where the quality and speed of scanners are both critical for market success, production cost of a CCD module is bound to increase and thus may lower its power to compete in the market.
Furthermore, in order to complete the exposure of CCD photodetectors within a short interval, the necessary light intensity of the light source must be increased. A light source having a high intensity is not only expensive, but also consumes more power and generates a higher surrounding temperature due to heating. Consequently, the working life of electronic devices will be shortened.
In addition, in order to shorten the exposure time and to increase scanning signal precision so that the quality and speed of operation of a scanner can be maintained, optical elements, CCD module and other related devices must be designed to have a high signal-to-noise (S/N) ratio. Consequently, the production cost of the scanner is raised.
In summary, a conventional light exposure control device has the following defects:
1. A high-intensity light source is required, leading to large power consumption and high operating temperature, thereby shortening the working life of electronic devices.
2. The signal-to-noise (S/N) ratio of optical elements and CCD modules has to be increased, thereby increasing production cost.
3. To capture a clear image from the CCD within a short interval requires highly sensitive photodetectors, thereby leading to an increase in production cost and a decrease in its ability to compete in the market.
In light of the foregoing, there is a need to improve light exposure control device.
Accordingly, the present invention provides a light exposure control device so that the length of exposure for each CCD photodetector can be extended without affecting scanning speed. In addition, the signal-to-noise (S/N) ratio can be increased without increasing the production cost of a CCD module. Therefore, competitiveness in the market will soar.
In a second aspect, the present invention provides a light exposure control device that permits the use of a lower-intensity light source so that power consumption and operating temperature can be reduced. Hence, working life of electronic components can be extended.
In a third aspect, the present invention provides a light exposure control device capable of using devices having a lower signal-to-noise (S/N) ratio so that production cost is lower and commercial value is higher.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a light exposure control device. The light exposure control device is capable of delivering three independent excitation signals from a counter so that a red photodetector, a green photodetector and a blue photodetector can be triggered separately. Since each excitation signal delivered to each CCD photodetector is independent from each other, and each CCD photodetector furthermore reacts to a specific excitation signal, the CCD photodetector is able to avert the effect of other unrelated excitation signals. Therefore, the length of exposure for each CCD photodetector is greatly increased. In addition, the excitation signals can be generated by an oscillator or a frequency generator.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.